In general, it is useful to be able to interchange the lenses of a pair of glasses, for example to interchange transparent lenses with tinted lenses, or corrective lenses with noncorrective lenses, and vice versa. It may also be useful, when taking part in certain sports, in particular skiing, to be able to interchange the lenses of a pair of glasses according to the weather.
There are pairs of glasses allowing interchanging of this sort. However, the existing models do not always allow this interchanging to be carried out simply and easily, do not always hold the lenses perfectly securely and do not always have a simple structure which is easy to manufacture.
There are also pairs of glasses comprising sides which can be held in the extended position, and which can be returned to their folded position when they are made to leave this extended position. Likewise, existing models having this feature are not always very simple and easy to manufacture.
Furthermore, existing pairs of glasses often have sides which either do not hold the pair perfectly on the user's head, or comprise ends which are relatively irritating to the ears.